Structure for cooling a surface

ABSTRACT

An apparatus for cooling a surface having a metal structure made of a material with high thermal conductivity, and designed to provide efficient cooling of the surface while minimizing mechanical stress between the metal structure and the surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/037,670, filed Jan. 18, 2005, now U.S. Pat No. 7,288,840, which isherein incorporated by reference in its entirety.

REFERENCE TO GOVERNMENT FUNDING

The present invention was made with Government support under ContractNo. H98230-04-C-0920, awarded by the Maryland Procurement Office. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to a structure for cooling, and more particularly,a flexible structure for attaching to a surface to be cooled.

BACKGROUND OF THE INVENTION

In silicon chip cooling, the heat generated by the chip is ultimatelyremoved by a heat sink, which is usually cooled by air or liquid. It iscrucial to have a path of high thermal conductivity between the chip andthe heat sink. The simplest form of chip cooling consists in a heat sinkmade of a high conductivity material, ideally copper, directly bonded tothe chip. However, direct bonding of a copper heat sink is limitedbecause of mechanical reliability issues. Due to the considerabledifference in thermal coefficients of expansion (TCE) of silicon andcopper, mechanical stress often leads to delamination or formation ofcracks between the silicon chip and the copper heat sink.

Several methods that have been used to address this problem include, forexample, interposing a thermal paste between the chip and the heat sinkto eliminate stress. A major drawback, however, is the low thermalconductivity of the paste. Another approach is to provide bondingbetween the copper heat sink and the silicon chip via an intermediatespreader (usually made of SiC) with TCE closely matched to silicon. Thisapproach adds to process complexity, costs, and non-optimized thermalconductivity.

A recent approach involves the use of a liquid metal layer, e.g.,gallium, or various alloys that include gallium, between the chip andthe heat sink. This is disclosed in U.S. patent application Ser. No.10/665798, “METHOD AND APPARATUS FOR CHIP COOLING”, filed on Sep. 18,2003. However, alternative cooling structures with improved thermalmatching and mechanical reliability are still needed.

SUMMARY OF THE INVENTION

One aspect of the invention provides an apparatus for cooling asubstrate, comprising a metal structure having a plurality of contactregions for attaching to the substrate, the metal structure defining atleast one channel for a fluid flow; where the plurality of contactregions defines an attachment area between the metal structure and thesubstrate that is substantially smaller than an area of contact betweenthe metal structure and the fluid flow; and an enclosure disposed overthe substrate and around the metal structure for confining the fluidflow.

Another aspect of the invention provides an apparatus for cooling asubstrate, comprising a metal structure made from a metal having a firstthermal coefficient of expansion, where the metal structure has a firstside for attaching to the substrate having a second thermal coefficientof expansion that is different from the first thermal coefficient ofexpansion, and the metal structure is configured to provide at least oneof sufficiently small dimensions and sufficient flexibility forrelieving mechanical stress arising from different thermal expansions ofthe substrate and the metal structure.

Another aspect of the invention provides a method of cooling asubstrate, comprising: providing a metal structure having at least onechannel, the metal structure being coupled to the substrate to define anattachment area between the metal structure and the substrate; providingan enclosure above the substrate and around the metal structure; flowinga fluid inside the enclosure and through the at least one channel of themetal structure such that the fluid contacts a surface area of the metalstructure that is substantially larger than the attachment area betweenthe metal structure and the substrate.

Yet another aspect of the invention provides a method of cooling asubstrate, comprising: providing a metal structure having at least onechannel, the metal structure being thermally coupled to the substrate,and configured to provide at least one of sufficiently small dimensionsand sufficient flexibility for relieving mechanical stress arising fromdifferent thermal expansions between the substrate and the metalstructure; providing an enclosure above the substrate and around themetal structure; and flowing a fluid coolant inside the enclosure andthrough the at least one channel of the metal structure.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe obtained by reference to the embodiments thereof which areillustrated in the appended drawings. It is to be noted, however, thatthe appended drawings illustrate only typical embodiments of thisinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1 illustrates one configuration of a heat sink structure with metalribbons and an enclosure for the heat sink structure;

FIG. 2 illustrates another configuration of a heat sink with metalribbons;

FIG. 3 illustrates a heat sink structure with metal mesh;

FIG. 4 illustrates a heat sink structure with slotted metal fins;

FIG. 5 illustrates a heat sink structure with metal pillars;

FIG. 6 illustrates a heat sink structure with metal fins; and

FIG. 7 illustrates a heat sink structure with a base plate.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

This invention relates to a structure for cooling a surface, and moreparticularly, a heat sink attached or bonded to a surface to allowefficient cooling while minimizing undesirable mechanical stress.Although this structure is particularly well-suited for cooling asilicon chip, it can generally be applied to cooling other surfaces orarticles. The structure, which is made of a material (may be asingle-component or multi-component material) with relatively highthermal conductivity, e.g., larger than about 200 W/mK, is designed withsufficient flexibility and/or small dimensions to maintain mechanicalintegrity and compliance between the surface to be cooled and thestructure, which may have a thermal coefficient of expansion that isvery different from the surface to be cooled. For convenience sake, thesurface to be cooled will also be referred to as the “hot” surface inthis discussion. Suitable materials for the heat sink include, forexample, metals such as copper, silver or aluminum, or a material suchas silicon carbide (SiC), aluminum silicon carbide (AlSiC), diamond orgraphite. Mixed sintered form of such materials, e.g., small particlesof diamond within a matrix of lower conductivity material such as AlSiC,can also be used to form the structure of this invention. Such amaterial (diamond particles within an AlSiC matrix) is availablecommercially with a thermal conductivity on the order of 600 W/mK(compared with thermal conductivities of about 1000 W/mK for singlecrystal diamond or graphite).

The structure, which can be implemented in several embodiments,generally has an “open” design—e.g., it has one or more channels orpathways within the structure to allow a fluid (or coolant) to flowthrough the structure. These channels or pathways may also be referredto as internal channels. The shapes and dimensions of the structure aredesigned to reduce mechanical stress arising from thermal expansionmismatch between the heat sink material and the hot surface, e.g., byproviding structural flexibility or mechanical compliance and/or havingrelatively small contact areas between the structure and the hotsurface. The structure is attached or bonded to the hot surface atselected or predetermined contact regions, and it is preferable that arelatively large fraction of the hot surface, e.g., at least about onehalf, is attached or coupled to the heat sink structure to facilitateefficient cooling. By keeping the contact area between the structure andthe hot surface to be substantially smaller than the surface area of thestructure exposed to the fluid flow (the “exposed” area), mechanicalstress can be reduced while still maintaining efficient heat transfer.Since the heat sink material has a relatively large thermalconductivity, the relatively small attachment or contact area will notsignificantly affect heat conduction from the hot surface.

During operation, a cooling fluid is allowed to flow through the one ormore channels of the structure. Suitable cooling fluids may include, forexample, water, glycol, freon, chlorofluoro-carbons and substitutes (orother refrigerants), Dynalene® heat transfer fluids (e.g., Dynalene HC,a water-based heat transfer fluid), butane, propane, hexane, and similaralkanes or other organic compounds, liquid metals such as alloys ofgallium-indium-tin, air or other gases, e.g., non-reactive or inertgases, nitrogen, helium, among others. By providing a relatively largesurface area (within a given volume occupied by the structure) ofcontact between the heat sink and the cooling fluid, efficient coolingof the heat sink, and thus, the hot surface, can be achieved. Optimalfluid utilization can be achieved by the use of a fluid flow under lowpressure over a large cooling surface area of the heat sink.

In one embodiment, the structure comprises metallic ribbons 104 directlyattached to a hot surface to be cooled. FIG. 1 a illustrates oneconfiguration in which the troughs of the ribbons 104 rest on asubstrate 102 (e.g., hot surface of a semiconductor device), with thebottom of each trough forming the contact areas 108 with the substrate102. In one example, the height (h) of the ribbons above the substrate102 and the width (w) of the ribbons are on the order of 1 mm, and eachribbon 104 has a thickness (t) on the order of 0.1 mm. In anotherexample, the height (h) of the ribbons is about five times the distance(d) between adjacent troughs. In other embodiments, the thickness of theribbon may range from about 0.05 mm to about 0.5 mm. The fluid flow 120is parallel to the substrate 102 and along a direction of low flowresistance. In one example, a cooling rate of about 20 W/cm²/C (Watt percentimeter square per degree centigrade) is achieved with a water flowrate of about 2 lpm (liter per minute) at a pressure of about 5 psi(pound per square inch). In general, flow rates within a range of about0.5 to about 5 lpm and a pressure in a range of about 1 to about 10 psiare sufficient to provide adequate cooling for semiconductor andintegrated circuits types of applications (e.g., with temperature rangeof about 50 to 100° C., power of about 50 to 500 W, and a semiconductorsubstrate area of about 0.5 to 5 cm²). The fluid is confined to a regionabove the substrate 102 by an enclosure 120, which is shownschematically in FIG. 1 a. The enclosure 120 can be attached to thesubstrate 102 using a variety of known methods to provide a leak-proofseal for fluid confinement on the sides. The enclosure 120 may have itsown inlet 122 and outlet 124 for directing the fluid flow to and fromthe substrate 102, or it may also be a part of a larger manifold (notshown) used to direct coolant fluid to this and possibly many othersubstrates. FIG. 1 b shows a cross-sectional view of the enclosure 120installed over the substrate 102 and around the metal ribbons 104, ormore generally, the heat sink structure. The enclosure 120 may beattached to the substrate 102 by any appropriate mechanical means, e.g.,using a clamp (not shown), with a gasket 130 at the bottom of theenclosure 120 to provide a leak-proof seal with the substrate 102. It isunderstood that enclosure 120, or variations thereof, may be used withother embodiments of the present invention to be described below.

In an alternative embodiment, an optional top sheet (or support sheet)110 may be provided for attaching to the “top” sides or troughs of themetallic ribbons 104. This configuration offers several advantages. Forexample, the manufacturing and packaging of the structure can befacilitated by pre-attaching one side of each metallic ribbon 104 to thetop sheet 110 before the entire structure is installed or attached tothe substrate 102. Additional mechanical stability can be achieved withthe top sheet 110 providing anchoring points for the metallic ribbons104. Furthermore, the top sheet 110 may serve as a top confinement forthe cooling fluid, in which case, it may also be a part of the enclosure120. In general, the top sheet 1O may be made from a variety ofmaterials, and can be attached to the metallic ribbons 104 using anyappropriate methods. When the top sheet 110 is made of a thermallyconductive material and attached with good thermal coupling to themetallic ribbons 104, it may also result in additional cooling by actingas part of the heat sink structure.

FIG. 2 illustrates another configuration of a heat sink structurecomprising metal ribbons 204, with the side of each ribbon contactingthe substrate 102 along the entire length of the ribbon. In this case,the contact area between each ribbon 204 and the substrate 102 is givenby the product of the thickness (t) and the length (l) of the ribbon204. During cooling operation, a fluid flows over the substrate 102along flow direction 220 through channels 206 defined by adjacentribbons 204. The relatively large surface area of contact between thefluid and the heat sink structure allows efficient heat transfer. Whenthe substrate expands in the x-direction, i.e., generally along thelongitudinal direction of the ribbons, the ribbons can expand like anaccordion, thereby relieving the stress along this direction that wouldotherwise occur due to mismatch in the thermal expansions. In thedirection perpendicular to x, the extent of the ribbons (approximately,the ribbon thickness) is small and does not build much stress over theshort distance. The periodicity of the ribbon's “wave” (i.e.,peak-to-peak or trough-to-trough spacing) is typically a millimeter. Asshown in the figures, ribbons 204 may also be attached to the optionaltop sheet 110. Furthermore, slots or cuts (not shown) may be provided atthe bottom sides (facing the substrate) of the ribbons for addedflexibility, if desired.

In the above embodiments, mechanical stress arising from the thermalexpansion mismatch between the ribbon material and the substrate 102 canbe relieved by one or more of the following features in the metalribbons—the curls or bends in the ribbons; the cuts or slots in theribbons; and the small thickness of the ribbons. The dimensions givenabove are illustrative for liquid cooling applications. In the case ofair or other gas cooling, the dimensions of the structures can be scaledappropriately according to the thermal conductivity. For example, sincethe thermal conductivity of air is two orders of magnitude smaller thanwater, the structure's dimension should be increased accordingly toachieve the desired cooling rate.

In another embodiment, the structure comprises metallic coils or meshes304, which may generally include woven or non-woven metallic threads,filaments, or strands. As shown in the example of FIG. 3, the portion ofmetallic meshes 304 contacting the substrate 102 is relatively smallcompared to the remaining portion that is not in contact with thesubstrate 102. A cooling fluid flows along a general direction 320through the open spaces or channels within the coils or meshes 304. Inthis example, metallic meshes 304 are made of thin copper wires, havinga diameter on the order of 0.1 millimeter, and a height of about 1 mmabove the substrate.

FIG. 4 illustrates another embodiment of a heat sink structure withslotted metal fins 404. The fins 404 contact the substrate 102 atcontact areas 408, and a plurality of slots 410 in the metal fins 404provide for flexibility or mechanical compliance in the structure, andrelieve the stress that would otherwise build-up over a long andcontinuous interface. In one embodiment, each fin has a thickness (t) ofabout 0.2 mm, a height (h) of about 1 mm, a spacing (s) of about 2 mmbetween slots, and a total length (l) that corresponds to the length ofthe chip (10 to 20 mm). During operation, a fluid coolant flows alongdirection 420, through channels or pathways 406 defined by adjacentmetal fins 404.

Two other embodiments are illustrated in FIGS. 5 and 6, which show heatsink structures with metal pillars 504 and metal fins 604, respectively,directly attached to the substrate 102. Cooling fluid flows over thesubstrate 102 generally along directions 520 and 620 through spaces orchannels defined by the pillar and fin arrangements. Although FIGS. 5-6show the pillars 504 and fins 604 as being arranged in regular orperiodic patterns, other arrangements, such as irregular patterns, arealso possible. However, some form of regular patterns, e.g., optimizedto provide maximize cooling and flow rate, is generally preferred. Thesepillars and fins are designed to be relatively small in size so that themechanical stress are minimized over the small bonded areas. In oneexample, each Cu pillar has a height of about 1 mm and a cross-sectionalarea of about 0.05 mm². In another example, Cu fins having a height ofabout 1 mm, thickness of about 0.2 mm and length of about 2 mm are used.In other embodiments, heights ranging from about 0.5 mm to about 5 mm,fin thickness (or width) or pillar diameter between about 0.05 mm toabout 0.5 mm, fin length from about 0.1 mm to about 2 mm may also beused. In these embodiments, even though the pillars or fins are fairlyrigid, the small dimensions of the pillars or fins, i.e., dimensions ofthe contact area with the hot surface, result in only a small differencein relative motion (arising from the different rates in thermalexpansion) between the substrate and the heat sink. Thus, the build upof stress (when the motion is constraint) is small. It should beunderstood that the dimensions of pillars and fins needed to provide forcertain desired degree of acceptable mechanical stress in a particularcooling application will vary according to the specific materials used.For a given substrate or chip, it is preferable that the total area ofthe heat sink structure being cooled, i.e., area exposed to the coolingfluid, be at least about five times the area of the substrate. It isalso preferable that this fluid-cooled or exposed area of the heat sinkstructure be at least 10 times the attachment area between the heat sinkand the substrate, although a factor of at least 5 times is alsoacceptable.

The structures described above are preferably directly attached—e.g.,soldered or bonded to the substrate or hot surface such as the backsideof a semiconductor chip. Soldering materials include a well known rangeof materials such as tin, indium, bismuth, silver, gallium, lead, amongothers, and their alloys. Bonding materials include organic adhesives,e.g., cyanolite and epoxies. Alternately, the structures (particularlyfins or pillars) can be directly grown by plating onto selected areas ofthe substrate. Metals such as Cu, Ni, Ag, and others are amenable toplating.

For each embodiment, the heat sink and the soldering material mayoptionally be covered with a material layer or coating to protect theheat sink or solder material from corrosion by the cooling fluid. In oneexample, the coating is a thermally conductive polymer such as Sylgard®Q3-3600, which is commercially available from Dow Corning. Depending onthe specific material and application needs, the protective coating mayhave a thickness ranging from about 1 μm to about 10 μm. Furthermore,although soldering materials that are compatible with the heat sinkmaterial are generally preferred, in situations where there may beconcerns of potential interaction between the soldering and the heatsink materials, a protective coating layer may also be used to limitsuch interactions. Such coating materials may include, for example,nickel, chromium, gold, platinum or combinations thereof.

In yet another embodiment, the heat sink structures described above mayalso be attached onto an interposed base plate, which is in turnattached to the substrate 102 using conventional methods such assoldering, bonding, liquid metal, thermal paste, . . . , etc. This isillustrated in FIG. 7, which shows a base plate 710 with metallicribbons attached to the top surface 712, and the bottom surface 714 ofthe base plate 710 attached to the substrate 102. The thermal andmechanical properties of the base plate 710 should be optimized toprovide efficient cooling and mechanical reliability. Not only does theuse of this base plate 710 facilitate manufacturing and packaging of theheat sink structures, it also allows the substrate 102 to be isolatedfrom the cooling fluid, thus alleviating concerns for potentialreliability issues for cooling substrates such as semiconductor chips.

For this embodiment, two general approaches may be used. One involvesthe use of a base plate 710 made of a material with thermal coefficientof expansion (TCE) closely-matched to that of the substrate. Forexample, in the case of a silicon substrate, the base plate 710 may bemade of silicon, silicon carbide, or invar steel, among others. Thethickness is preferably large enough to provide sufficient mechanicalintegrity, but thin enough to allow for high thermal conductivity. Forexample, thickness values can range from about 0.05 to 0.5 millimeter.

Another approach involves using a base plate 710 that is sufficientlythin so that the mechanical stress produced is acceptable, despite alarger mismatch of TCE with the substrate 102. For example, in the caseof a silicon substrate, the base plate 710 may be a thin foil of copper,silver or nickel, and a thickness less than about 0.03 millimeter willgenerally be acceptable.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise other embodiments withoutdeparting from the basic scope of the present invention.

1. An apparatus for cooling a heat generating surface, the heatgenerating surface having a first thermal coefficient of expansion,comprising: a metal structure made from a metal having a second thermalcoefficient of expansion that is different from the first thermalcoefficient of expansion; and an enclosure disposed above the heatgenerating surface and around the metal structure, wherein the metalstructure has a first side for attaching directly to the heat generatingsurface and a second side attached to a support sheet such that themetal structure is disposed between the support sheet and the heatgenerating surface, wherein the metal structure is configured to provideat least one of sufficiently small dimensions and sufficient flexibilityfor relieving mechanical stress arising from different thermalexpansions of the heat generating surface and the metal structure, andwherein the metal structure has an attachment area with the heatgenerating surface that is substantially smaller than a total surfacearea of the metal structure.
 2. The apparatus of claim 1, wherein themetal structure comprises at least one ribbon.
 3. The apparatus of claim1, wherein the metal structure comprises at least one coil.
 4. Theapparatus of claim 1, wherein the metal structure comprises at least onemesh.
 5. The apparatus of claim 1, wherein the metal structure comprisesat least one pillar.
 6. The apparatus of claim 5, wherein the at leastone pillar comprises a plurality of pillars arranged in a periodicpattern.
 7. The apparatus of claim 1, wherein the metal structurecomprises at least one fin.
 8. The apparatus of claim 7, wherein the atleast one fin comprises a plurality of fins arranged in a periodicpattern.
 9. The apparatus of claim 1, wherein the metal structurecomprises agglomerated metal particles.
 10. The apparatus of claim 1,wherein the metal structure comprises an expandable accordion-shapedstructure.
 11. The apparatus of claim 1, wherein at least approximatelyhalf of a surface of the heat generating surface is attached to themetal structure.
 12. The apparatus of claim 1, further comprising: acooling fluid flowing through one or more channels defined by the metalstructure.
 13. The apparatus of claim 1, wherein the enclosure furthercomprises: an inlet for providing a cooling fluid to the heat generatingsurface; and an outlet for removing the cooling fluid from the heatgenerating surface.
 14. The apparatus of claim 1, wherein the metalstructure further comprises: one or more slots formed in proximity tothe first side of the metal structure.
 15. The apparatus of claim 1,wherein the metal structure is soldered to the heat generating surface.16. The apparatus of claim 1, wherein the metal structure is bonded tothe heat generating surface.
 17. The apparatus of claim 1, wherein themetal structure is coated with a thermally conductive polymer.
 18. Theapparatus of claim 1, wherein the enclosure is also disposed around thesupport sheet.